In order to develop portable small-scale projectors smaller than a palm size or to commercialize a projector that is small enough to be embedded in a notebook or the like, a projector having a small size and low power consumption should be developed. In order to achieve a small-scale and low-power projector, a small-scale light source with excellent power consumption should be. The most suitable light source for such a low power projector is a laser light source or an electroluminescent device (LED).
LED light sources and laser light sources are known as efficient light sources that emit high intensity light at low power. However, both devices have disadvantages. The LED light source has a disadvantage in that it has a lower light efficiency than the laser as a light source having a high etendue. In contrast, the laser light source exhibits higher luminous efficacy than the LED, but has a disadvantage in that the laser has a unique interference pattern called speckle. Therefore, if the speckle effect of the laser light can be reduced, there may be no small light source more suitable for a micro-projector than the laser light source.
FIG. 1 shows the structure of a projector using a conventional laser light source implemented with a reflective optical modulator. The projector using the conventional laser light source includes a laser light source composed of an R light source 10R, a G light source 10G and a B light source 10B, a dichroic mirror 50 composed of 50R, 50G and 50B, reflectors 51 and 53, a diffuser 20, a beam shaper 30, a field lens 40, an optical modulator 60, a projection lens 70, and a polarized beam splitter 80.
The laser light source preferably sequentially emits R/G/B light. Sequentially emitting light from the laser R, G and B light sources 10R, 10G and 10B means that, when the total time needed to emit a whole frame is T, the R light source emits light for T/3, the G light source emits light for the subsequent T/3, and the G light source emits light for the next T/3.
Since a light source for a micro-projector is required to have a small size and a high light output power, a laser light source or an electroluminescent device (LED) is a suitable light source. At least one laser light source from among the three light sources 10R, 10G, and 10B should be included. Preferably, all three laser light sources are used, or a hybrid type light source is configured using laser and LED light sources, if necessary. Light emitted from the three light sources is reflected by or transmitted through respective dichroic mirrors 50R, 50G, and 50B, and is incident on the diffuser 20. The dichroic mirror 50G functions to reflect light emitted from the G light source 10G (green laser light) and allow light from the other light sources to be transmitted therethrough. As the dichroic mirror 50G, a typical mirror that reflects all visible wavelengths can be employed. The dichroic mirror 50R functions to reflect light emitted from the R light source 10R (red laser light) and allow light of other wavelengths to be transmitted therethrough. The dichroic mirror 50B functions to emit light from the B light source 10B (blue laser light) and allow light in the other wavelength range to be transmitted therethrough.
The laser light emitted from the laser light sources 10R, 10G, and 10B is reflected by the reflectors 51 and 53, and then is incident on the diffuser 20. The diffuser vibrates perpendicular to the optical axis, and thus randomness of light increases as the light passes through the diffuser. The diffuser, which is a device for removing speckles unique to a laser beam, lowers coherence of the laser beam to reduce speckles. Vibrating the diffuser 20 may further reduce the speckles.
The beam shape of the light passing through the diffuser 20 is deformed by the beam shaper 30. Deforming the beam shape is intended to form a beam shape that is suitable for the shape of the surface of incidence of the optical modulator 60 to improve luminous efficacy.
A typical example of the beam shaper 30 may be a fly-eye lens. The fly-eye lens is implemented using a plurality of small lens bodies. The small lens bodies implemented in the fly-eye lens may have various shapes. For example, they may have a rectangular convex shape, a hexagonal convex shape, a circular shape, or the like, but preferably have the same shape as that of the optical modulator (more specifically, the effective screen shape of the optical modulator). For example, if the effective screen shape of the optical modulator is rectangular, the small lens bodies may be formed in a rectangular shape, thereby minimizing optical loss.
In the embodiment of FIG. 1, a double-sided fly-eye lens having small lens bodies on both sides thereof is used as the beam shaper 30. However, two single-sided fly-eye lenses may be used. The multiple small lens bodies formed on both sides or on two lenses have a one-to-one correspondence relationship with each other.
The field lens 40 serves to focus the light shaped by the beam shaper 30 onto the optical modulator 60. The field lens 40 is generally comprised of one to three lenses and accurate focusing may be achieved by adjusting the distance between these lenses and the beam shaper 30.
The optical modulator 60 refers to a device that selectively transmits or blocks incident light or changes an optical path to generate an image. Typical examples of the optical modulator 60 may include a digital micromirror device (DMD), a liquid crystal display device (LCD) and an LCOS. The DMD is a device used in a DLP projector using as many digital mirrors arranged in a matrix form as the pixels based on field sequential. The DLP is a projector that implements gradation and an image by reflecting light emitted from a light source onto a screen by adjusting a light path according to a digital mirror. The LCD is a device that selectively turns on/off liquid crystals to generate an image. Projectors using the LCD include a direct type projector, a projection type projector, and a reflective projector. The direct type projector causes light from the backlight behind the LCD to pass through the liquid crystal panel such that a generated image is directly viewed. The projection type projector enlarges an image generated when light passes through the LCD by using a projection lens and then projects the enlarged image onto a screen such that the image reflected onto the screen is viewed. The reflective projector has almost the same structure as the projection type projector, except that a reflective film is formed on a lower substrate such that light is reflected and projected onto the screen with the image enlarged. LCoS (Liquid Crystal on Silicon) is a reflective liquid crystal display, which is a reflective optical device that operates using a silicon substrate employed in place of transparent glass, which is conventionally used as the lower substrate of the two substrates of an LCD.
In the embodiment of FIG. 1, the polarized beam splitter 80, which is a reflective optical system, serves to transmit an image generated by the optical modulator 60 to the projection lens 70. The polarized beam splitter (PBS) 80, which has a polarization splitting film diagonally arranged in a hexahedral body formed of glass, is an essential optical device for a reflective optical engine.
The polarized beam splitter 80 is an optical device that uses a polarization splitting film to allow P-polarized light to pass therethrough and reflect S-polarized light in a direction opposite to the projection lens. Alternatively, the polarized beam splitter may be configured to pass S-polarized light and to reflect P-polarized light, if necessary. In FIG. 1, for simplicity, the polarized beam splitter is illustrated as passing P-polarized light and reflecting S-polarized light. Therefore, to maintain luminous efficacy, the light emitted from the laser light source 10 should be converted into a linearly polarized state at a certain point on the optical path.
The P-polarized light passed through the polarization splitting film of the polarized beam splitter 80 is converted into S-polarized light in the process of forming an image through the reflective optical modulator 60, and the image light converted into the S-polarized light is again incident on the polarized beam splitter 80 and reaches the polarization splitting film. Since the image light is S polarized light at this time, the entire image light is reflected on the polarization splitting film and is incident on the projection lens 70. The projection lens 70 is configured using multiple lenses, and serves to enlarge and project an image formed by the optical modulator 60 onto a screen (not shown). A projector using the conventional laser light source shown in FIG. 1 has high luminous efficacy and a wide color range, but it generates an interference pattern known as speckle in the enlarged projection image. This projector cannot be used as a high-quality projector.